Vertex bevel incorrectly flips normals when used on just an isolated spline of vertices #66320

New Issue

Jesse Yurkovich · 2019-07-02T02:51:20+02:00

Jesse Yurkovich commented

2019-07-02 02:51:20 +02:00

System Information
Operating system: Windows-10-10.0.18362 64 Bits
Graphics card: Quadro 600/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 369.09

Blender Version
Broken: version: 2.80 (sub 74), branch: fixes (modified), commit date: 2019-07-01 18:05, hash: c2a762c8e6
Worked: (optional)

Short description of error
Start with an isolated spline of vertices -- one in which all the edges have consistent normals (using a screw modifier to revolve the spline around a midpoint will allow you to visualize the surface normals in this case).

If you attempt to bevel 1 or more vertices on this object, the normals will incorrectly flip to one side of the bevel. This is very problematic; especially now since the subdivision surface modifier is sensitive to the normal direction.

Before bevel:

After bevel:

Exact steps for others to reproduce the error
vert-bevel-flip.blend

Load attached .blend
- It contains a spline with 6 vertices, revolved around its origin (to create a surface)
- It has also enabled the Face Orientation overlay
Attempt to vertex bevel one or more of the vertices in the middle
Notice that the normals become flipped -- with no ability to recalculate (recalculate only seems to work on faces, not this spline)

**System Information** Operating system: Windows-10-10.0.18362 64 Bits Graphics card: Quadro 600/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 369.09 **Blender Version** Broken: version: 2.80 (sub 74), branch: fixes (modified), commit date: 2019-07-01 18:05, hash: `c2a762c8e6` Worked: (optional) **Short description of error** Start with an isolated spline of vertices -- one in which all the edges have consistent normals (using a screw modifier to revolve the spline around a midpoint will allow you to visualize the surface normals in this case). If you attempt to bevel 1 or more vertices on this object, the normals will incorrectly flip to one side of the bevel. This is very problematic; especially now since the subdivision surface modifier is sensitive to the normal direction. Before bevel: ![before-vert-bevel.png](https://archive.blender.org/developer/F7559571/before-vert-bevel.png) After bevel: ![after-vert-bevel.png](https://archive.blender.org/developer/F7559573/after-vert-bevel.png) **Exact steps for others to reproduce the error** [vert-bevel-flip.blend](https://archive.blender.org/developer/F7559575/vert-bevel-flip.blend) - Load attached .blend - It contains a spline with 6 vertices, revolved around its origin (to create a surface) - It has also enabled the Face Orientation overlay - Attempt to vertex bevel one or more of the vertices in the middle - Notice that the normals become flipped -- with no ability to recalculate (recalculate only seems to work on faces, not this spline)

Jesse Yurkovich commented

2019-07-02 02:51:20 +02:00

Added subscriber: @deadpin

blender-admin commented

2019-07-02 02:51:20 +02:00

#82215 was marked as duplicate of this issue

Germano Cavalcante commented

2019-07-02 17:53:59 +02:00

Added subscribers: @mont29, @mano-wii

Bastien Montagne was assigned by Germano Cavalcante

2019-07-02 17:53:59 +02:00

Germano Cavalcante commented

2019-07-02 17:53:59 +02:00

@mont29, the same behavior was seen in blender 2.79, so it is not a regression. Feel free to set the priority.

Bastien Montagne removed their assignment 2019-07-02 19:59:44 +02:00

Howard Trickey was assigned by Bastien Montagne

2019-07-02 19:59:44 +02:00

Bastien Montagne commented

2019-07-02 19:59:44 +02:00

Added subscriber: @howardt

Bastien Montagne commented

2019-07-02 19:59:44 +02:00

This is for @howardt, he is maintainer of the bevel code. :)

Howard Trickey commented

2019-07-03 13:53:30 +02:00

This is a problem with the screw modifier, not bevel.
Edges don't have normals, so this is not a matter of bevel somehow changing the normals of edges.
What does change is the ordering of the vertex indices. This should not matter to the screw modifier -- it should try to make consistent normals no matter what the ordering of the vertices, and it would be too hard a constraint on tools like bevel to try to maintain a consistent vertex ordering (because, for instance, two new vertices are created here), and in general, other tools and modifiers should be made to work despite the internal vertex index ordering.
I will note that a similar problem happens if instead of using a screw modifier, one does an extrude on this line of vertices after beveling a vertex in the middle.

This is a problem with the screw modifier, not bevel. Edges don't have normals, so this is not a matter of bevel somehow changing the normals of edges. What does change is the ordering of the vertex indices. This should not matter to the screw modifier -- it should try to make consistent normals no matter what the ordering of the vertices, and it would be too hard a constraint on tools like bevel to try to maintain a consistent vertex ordering (because, for instance, two new vertices are created here), and in general, other tools and modifiers should be made to work despite the internal vertex index ordering. I will note that a similar problem happens if instead of using a screw modifier, one does an extrude on this line of vertices after beveling a vertex in the middle.

Howard Trickey commented

2019-07-03 15:21:54 +02:00

Changed status from 'Open' to: 'Resolved'

Howard Trickey closed this issue

2019-07-03 15:21:54 +02:00

Howard Trickey commented

2019-07-03 15:21:54 +02:00

I looked at the screw modifier code, and noted that it had code to handle this properly -- it just has to be enabled by enabling the 'Calc order' option in the modifier. So this resolves the problem reported in this bug.
(There remains the similar problem with Extrude, but since that likely needs code like 'calc order' stuff in the screw modifier, which is a fairly extensive change, I will leave that as a future TODO.)

I looked at the screw modifier code, and noted that it had code to handle this properly -- it just has to be enabled by enabling the 'Calc order' option in the modifier. So this resolves the problem reported in this bug. (There remains the similar problem with Extrude, but since that likely needs code like 'calc order' stuff in the screw modifier, which is a fairly extensive change, I will leave that as a future TODO.)

Richard Antalik commented

2020-10-29 14:00:53 +01:00

Added subscribers: @rajq123, @iss

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Vertex bevel incorrectly flips normals when used on just an isolated spline of vertices #66320